1. Field of the Invention
The present invention pertains to a linear motor having a field magnet stator for propulsion and a movable piece having an armature coil.
2. Description of the Related Art
Conventionally, in connection with this type of linear motor, coil bobbin 201 as shown in FIG. 1A is used in the manufacture of a movable piece, and armature coil 202 is mounted on this bobbin 201 as shown in FIGS. 1B and 1C.
Bobbin 201 in FIG. 1A comprises round flanges 201b located on either end of cylindrical part 201a. Armature coil 202 is formed between said flanges around cylindrical part 201a. Furthermore, cylindrical movable piece yoke 203 is attached around armature coil 202. Movable piece 20 is formed in this way, and is placed, using its bobbin 201, over a rod-like stator 10 having a round cross-sectional configuration and having field magnet 101, providing a linear drive motor.
Armature coil 202 of movable piece 20 comprises coils having different electric phases, which are arranged such that there is no gap on movable piece 20 in FIG. 1B. The coils on movable piece 20 in FIG. 1C, however, are arranged such that there is a gap between the coils.
Position detecting sensors that detect the current position of the movable piece relative to the field magnet are normally attached to the movable piece of a linear drive motor in order to control the electric current of the coils of different phases that comprise the armature coil in accordance with their positions so that constant propulsion may be obtained regardless of the position of the movable piece. Hall elements, which are magnetic-electric conversion elements, are most often used as such position detecting sensors. A Hall element senses the strength of the magnetic field generated by the field magnet. Movable piece propulsion may be continuously obtained regardless of the position of the movable piece by applying to the coil electric current having a volume and orientation corresponding to the strength of the magnetic field sensed by the Hall element.
In movable piece 20 shown in FIG. 1B, Hall elements h are attached to the outer surface of the armature coil such that they correspond to the coils of different phases. In movable piece 20 shown in FIG. 1C, Hall elements h are attached to the cylindrical part 201a of bobbin 201 such that they are sandwiched by the coils. In many cases, a sensor that detects the scale of an encoder that is used to detect the distance over which the movable piece traveled or to detect or control the speed of the movable piece (hereinafter called a scale detecting sensor) is used with the movable piece of a linear drive motor, in addition to the position detecting sensors described above.
An optical encoder, magnetic encoder, etc. is used as the encoder. When an optical encoder is used, the scale detecting sensor is an optical sensor and the scale is an optical scale that provides signals that can be detected by said sensor. When a magnetic encoder is used, the scale detecting sensor is a magnetic sensor (a representative example would be a sensor using a magnetic resistance element called an MR element) and a fine magnetized member comprising magnetic poles arranged with a small pitch (i.e., a magnetic scale) is used for the scale. The scale detecting sensor of such a construction is also placed at an appropriate position on the movable piece.
However, as shown in FIG. 1B, when sensors such as Hall elements and MR elements that detect magnetic information are placed on the outer surface of the armature coil, the distance between the sensor and the members that provide magnetic information such as the field magnet and magnetic scale on the stator at the center of the armature coil becomes large, which reduces the sensitivity, as well as the output signal power, of the sensors. As a result, it becomes necessary to amplify the signals, or to increase the amplification rate, as a result of which the problem of noise becomes more marked.
As shown in FIG. 1C, where sensors that detect magnetic information are located between the coils, the sensitivity of the sensors improves because they can be made closer to the members on the stator that provide magnetic information. However, since the coils are located with a gap in between, the drive motor itself becomes large in size, and since it is necessary to support the coils with the gap in between, the configuration of the coil bobbin that holds the coils becomes large and/or complex.
In addition, where an optical encoder is used, the scale detecting sensor must be placed close to and directly facing the optical scale. Therefore, when the optical scale is placed on the stator, the scale detecting sensor cannot be located on the outer surface of the armature coil or between the coils of different phases, and must be located away from the armature coil. In this case, the stator becomes large and complex to the same extent.